In hybrid fiber coax (hereinafter sometimes HFC) and RF over glass (hereinafter sometimes RFOG) CATV (hereinafter sometimes cable) plant designs, there is a forward path (from the cable company's transmission point to the subscriber's premises) and a return path (from the subscriber's premises back to the cable company's receiving point). The forward path is a trunk-to-branch topology wherein signals are split and split until they reach the subscriber's premises. In contrast, the return path is a combining branch-to-trunk where signals combine until they reach the cable headend. HFC is glass fiber from the headend to distribution points where it is converted to RF, and then RF amplifiers and stretches of coaxial cable to the subscriber's premises. RFOG is basically fiber optic cable to the subscriber, although at the subscriber's premises, the downstream-bound optical signal is converted to RF for decoding by the subscriber's display apparatus, computers, and the like, and, conversely, the upstream-bound signal is converted from RF to optical for transport.
Since subscribers' premises are not the property of the cable operator and signals from many subscriber's premises on a particular node combine in the return band, this section of the cable operator's plant is most subject to noise as every subscriber's premises acts like an antenna and noise accumulates, as in a funnel, until it reaches the cable headend.
Many systems have been designed to assist cable operators to monitor their return band frequency spectrum (for example, 5 to 42 MHz, 5 to 65 MHz, 5 to 85 MHz, or even 5 to 200 MHz) for noise. When noise above a certain threshold is detected, alarms are typically generated, and personnel are dispatched to repair the problem.
In an HFC network, technicians move from branch to branch looking for the noise, narrowing it down until the source is found. In an RFOG network, technicians must go directly to each subscriber's premises individually to search for noise problems. The reason for this is that RFOG has fiber optic cable all the way to the subscriber's premises. There is no RF signal to test except at the subscriber's premises. This makes finding noise problems originating at subscriber's premises somewhat more time consuming and expensive in RFOG architectures.